This invention relates generally to electrostatographic reproduction machines, and more particularly to a machine including a belt position changing mechanism for reducing sheet edge wear defects from a contact belt fusing apparatus therein.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roller or to a latent image on the photoconductive member. The toner attracted to a donor roller is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rollers with the toner image contacting the heated fuser roller to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser Roller (NFFR), the heated fuser roller is provided with a layer or layers that are deformable by a harder pressure roller when the two rollers are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll.
The heated fuser roller is usually the roller that contacts the toner images on a substrate such as plain paper. In any event, the roller contacting the toner images is usually provided with an adhesive (low surface energy) material for preventing toner offset to the fuser member. Three materials which are commonly used for such purposes are PFA, Viton.TM. and silicone rubber.
Roller fusers work very well for fusing color images at low speeds since the required process conditions such as temperature, pressure and dwell can easily be achieved. When process speeds approach 100 pages per minute (ppm) roller fusing performance starts to falter. At such higher speeds, dwell must remain constant which necessitates an increase in nip width. Increasing nip width can be accomplished most readily by either increasing the fuser roller (FR) rubber thickness and/or the outside diameter of the roll. Each of these solutions reach their limit at about 100 ppm. Specifically, the rubber thickness is limited by the maximum temperature the rubber can withstand and the thermal gradient across the elastomer layer. The roller size becomes a critical issue for reasons of space, weight, cost, & stripping.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
U.S. Pat. No. 5,250,998 granted to Ueda et al on Oct. 5, 1993 discloses a toner image fixing device wherein there is provided an endless belt looped up around a heating roller and a conveyance roller, a pressure roller for pressing a sheet having a toner image onto the heating roller with the endless belt intervening between the pressure roller and the heating roller. A sensor is disposed inside the loop of the belt so as to come in contact with the heating roller, for detecting the temperature of the heating roller. The fixing temperature for the toner image is controlled on the basis of the temperature of the heating roller detected by the sensor. A first nip region is formed on a pressing portion located between the heating roller and the fixing roller. A second nip region is formed between the belt and the fixing roller, continuing from the first nip region but without contacting the heating roller.
U.S. Pat. No. 5,465,146 granted to Hgashi et al on Nov. 7, 1995 relates to a fixing device to be used in electrophotographic apparatus for providing a clear fixed image with no offset with use of no oil or the least amount of oil, wherein an endless fixing belt provided with a metal body having a release thin film thereon is stretched between a fixing roller having a elastic surface and a heating roller, a pressing roller is arranged to press the surface of the elastic fixing roller upwardly from the lower side thereof through the fixing belt to form a nip portion between the fixing belt and the pressing roller, a guide plate for unfixed image carrying support member is provided underneath the fixing belt, between the heating roller and the nip portion, to form substantially a linear heating path between the guide plate and the fixing belt, and the metal body of the fixing belt has a heat capacity per cm.sup.2 within the range of 0.001 to 0.02 cal/.degree. C.
A problem encountered with heat and pressure fusers or fusing apparatus is fusing edge wear, or defects caused to the fusing surface by the edges of sheets being fused repeatedly along the same path through the fusing nip. Such paper or sheet edge wear of the fusing surface caused by such continuous paper or sheet edge contact in the exact same place ordinarily result in significant image defects when larger media is forced to travel over this worn edge. Such fusing surface edge wear is a significant problem for both roller and belt fusing apparatus. Although there may be disagreement as to the exact mechanical cause of edge wear, where it occurs, and its undesirable effects are easy to tell. It occurs on the fusing surface of a fusing belt or roller at the points of contact between the edges of the substrate being fused and the outer usually compliant surface layer of the fusing belt or roller. In most cases, fusing edge wear as such is seen and felt as a small groove around the roller or along the surface of the belt.
The amount of edge wear on any fusing surface ordinarily is directly related to the volume of a particular size substrate that is run using such surface. For example, when mostly 8.5".times.11" (short edge feed) paper is run and fused through a machine at high volumes, it will cause two edge related wear marks, approximately 8.5" apart on the fusing surface. When 11".times.17" paper is run and fused (short edge feed) through the machine, each of these two marks then shows up as a line defect on the 11".times.17" fused image copy sheet, if the sheets are center registered. Edge registration of the sheets results only in one such line defect.